Single panel color image projection system

ABSTRACT

A color projection system includes a lamp and at least one diffraction grating or at least one prism that diffracts light from the lamp onto an LC microdisplay panel. By moving either the at least one diffraction grating or the at least one prism with respect to the incident light from the lamp, the diffracted light received by the LC microdisplay panel sequentially comprises red, green, and blue light.

TECHNICAL FIELD

This invention relates generally to color image projection systems, andmore particularly to color image projection systems needing just asingle liquid crystal (LC) microdisplay panel.

BACKGROUND

Color image projection systems that use liquid crystal (LC) microdisplaypanels to modulate light projected onto a screen such as used inlarge-screen televisions face a number of technical challenges as wellas manufacturing cost challenges. These challenges may be furtherexplained with respect to a conventional LC-microdisplay-panel-basedcolor image projection system. In such a system, there must be a lightsource that is then modulated by a light valve containing one or more LCmicrodisplay panel(s) before the resulting modulated-light is projectedonto a screen for viewing by a user. To achieve the desired viewablecolor image, three primary colors (typically red (R), green (G), andblue (B)) are separated from the white light provided by the lightsource. These primary colors are separately modulated by one or morelight valves and then recombined or superimposed to form the image.

Because the image is formed from the separately-modulated beams of RGBcolored light, there must be some means of separating these colors fromthe white light provided by the light source. These color separationmeans include dichroic mirrors, prism cubes, and color wheels. The lightvalve may contain three LC microdisplay panels: one to modulate the redlight, one to modulate the green light, and one to modulate the bluelight. Alternatively, the light valve may have just a single LCmicrodisplay panel that simultaneously modulates the RGB light usingsubpixels (one subpixel for each color being modulated). More recently,Philips has proposed a single LCOS (liquid crystal on silicon)microdisplay panel projection system using a scheme called “scrollingcolors”—that is, after RGB color separation, three rotating prisms areemployed to “scroll” RGB colors in field sequence respectively for thered, green and blue beams. The scrolling RGB beams are then realignedand modulated by a single microdisplay panel before projected onto thescreen.

Regardless of whether the light valve contains one or three LCmicrodisplay panels, a number of problems arise in the design of suchconventional color projection systems. For example, the use of threedichroic mirrors increases component cost and introduces the problem ofrealigning the separated RGB signals. Any misalignment will blur and/orintroduce color shifts on the projected images. Alternatively, if colorwheels are used to separate the RGB light, substantial power losses areintroduced, inhibiting effective use of the light source. Moreover,should three separate LC microdisplay panels be used to individuallymodulate the separated RGB light beams, expensive and cumbersomealignment lenses are necessary to realign the separately-modulated lightbeams into a single RGB image, adding to the expense of providing threeLC microdisplay panels. Furthermore, if just a single LC microdisplaypanel containing RGB subpixels is used to simultaneously modulate thered, green, and blue light beams, expensive and cumbersome alignmentlenses are still necessary to direct the beams exactly to the respectiveRGB subpixels. In addition, a single LC microdisplay panel with eachpixel containing R,G, and B colored subpixels requires a complicateddesign and manufacturing process which reduces the overall product yieldand increases cost. The more advanced Philips single LCOS panel designstill has the problem of realigning the separated RGB beams and theassociated complicated optics and increased cost.

Accordingly, there is a need in the art for improved color imageprojection systems having simplified optics allowing a more efficientuse of the light source without the necessity of realigning andrecombining separate RGB colored images.

SUMMARY

In accordance with one aspect of the invention, a color projectionsystem is provided that includes a lamp; at least one diffractiongrating configured to diffract light from the lamp into a diffractedbeam; and an LC microdisplay panel configured to modulate the diffractedbeam from the at least one diffraction grating, wherein by moving the atleast one diffraction grating with respect to the light from the lamp,the diffracted beam sequentially comprises a red, a green, and a bluebeam.

In accordance with another aspect of the invention, a color projectionsystem is provided that includes a lamp; at least one prism configuredto diffract light from the lamp into a diffracted beam; and an LCmicrodisplay panel configured to modulate the diffracted beam from theat least one prism, wherein by moving the at least one prism withrespect to the light from the lamp, the diffracted beam sequentiallycomprises a red, a green, and a blue beam.

In accordance with another aspect of the invention, a method is providedthat includes the acts of providing a light beam incident on at leastone diffraction grating; moving the at least one diffraction gratingwith respect to the incident light beam, wherein the movement of the atleast one diffraction grating is such that a diffracted light beam fromthe at least one diffraction grating sequentially comprises red, green,and blue light; and modulating the sequentially-provided red, green andblue light received by the LC microdisplay panel to project an imageonto a screen.

In accordance with yet another aspect of the invention, a method isprovided that includes the acts of providing a light beam incident on atleast one prism; moving the at least one prism with respect to theincident light beam, wherein the movement of the at least one prism issuch that a diffracted light beam from the at least one prismsequentially comprises red, green, and blue light; and modulating thesequentially-provided red, green and blue light received by the LCmicrodisplay panel to project an image onto a screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a color image projection system using a single LCmicrodisplay panel and a reflection grating according to one embodimentof the invention.

FIG. 2 a illustrates the relationship between an incident light bean andthe first order diffracted beams for a reflection grating.

FIG. 2 b illustrates the relationship between an incident light beam andthe first order diffracted beams for a transmission grating

FIG. 3 illustrates the typical spectral power for an ultra high pressurelamp.

FIG. 4 is a diagram of a color image projection system using a single LCmicrodisplay panel and a transmission grating according to oneembodiment of the invention.

FIG. 5 illustrates a synchronous relationship between the driving imagedata for the single LC microdisplay panel of FIGS. 1 and 4 and thedesired separated R, G, and B beams that are projected onto the singleLC microdisplay panel.

FIG. 6 illustrates a motor configured to rock the diffraction grating ofFIGS. 1 and 4.

FIG. 7 is a diagram of a color image projection system using a single LCmicrodisplay panel and a dispersive prism according to one embodiment ofthe invention.

FIG. 8 a is a top view of an arrangement of three diffraction gratingsfor separating RGB light according to one embodiment of the invention.

FIG. 8 b is a side view of the arrangement of three diffraction gratingsof FIG. 8 a.

Embodiments of the present invention and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

FIG. 1 illustrates a color projection system 100 that improves theinherent losses typically experienced during separation of white lightinto red, green, and blue light beams. Lamp 105, which may be an ultrahigh pressure (UHP) lamp or other suitable lamp, provides a beam ofwhite light. A single LC microdisplay panel 110 receives red, green, andblue light beams separated from the white light provided by lamp 105. LCmicrodisplay panel 110 modulates these beams as is known in the art sothat pixels for an image projected onto screen 120 comprise theappropriate values of red, green, and blue light, thereby achieving thedesired colors for each pixel. In the embodiment shown, LC microdisplaypanel 110 combines with polarization beam splitter 130 in a reflectiveconfiguration. It will be appreciated, however, that a transmissiveconfiguration for LC microdisplay panel 110 may also be used as is knownin the art. Regardless of whether LC microdisplay panel 110 isconfigured in a reflective or transmissive configuration, LCmicrodisplay panel 110 may be in either an on or off axis as is alsoknown in the art.

A diffraction grating 140 may be used to diffract a white light beam 145provided by lamp 105 into a diffracted beam. In the embodimentillustrated, diffraction grating 140 is a reflective grating. However,it will be appreciated that diffraction grating 140 could also be atransmission grating as will be explained further herein. The physics ofdiffraction gratings are well known. For example, consider FIG. 2 a,which illustrates the relationship between an incident light beam 200and a first order refracted beam 205 and also a negative first orderrefracted beam 210 for a reflective diffraction grating 220. Therelationship between diffracted beams 205 and 210 and incident beam 200is determined by the spacing “d” between grooves formed on the surfaceof diffraction grating 220. This relationship is given mathematicallyby:mλ=d(sinα+sinβ)  Equation 1where the angles α and β are defined with respect to a normal 225 to theplane defined by diffraction grating 220, λ is the wavelength of theincident light beam, and m is an integer value defining the diffractionorder. Angles α and β have a positive or negative value depending uponwhich side of normal 225 the resulting diffracted beam lies as shown bythe positive and negative signs adjacent normal 225. To show thediffraction order of the refracted beam, the angle β is given asubscript. From Equation 1 and FIG. 2 a, it can be seen that given theangle α and the frequency of incident light beam 200, the angles β₁ andβ⁻¹ are defined for the diffracted beams 205 and 210, respectively.

Referring back to FIG. 1, the angles α and β are also shown. Forgenerality, the diffraction order m for β is not shown. In general,although any diffraction order could conceivably be used, a value of m=1or m=−1 provides the greatest power in diffracted beam 150. Thus, itwill be assumed that β corresponds to either diffraction order in thefollowing discussion. Light beam 145 incident on diffraction grating 140is not a monochromatic beam such as incident beam 200 discussed withrespect to FIG. 2 a. Instead, white light beam 145 will typically have aspectrum 300 such as that shown in FIG. 3. Examination of spectrum 300shows that the brightness power (in milliwatts per nanometer ofwavelength) is concentrated in the blue, green, and red wavelengths anddenoted by the B, G, and R letterings, respectively. The blue wavelengthcorresponds to approximately 450 nanometers in wavelength whereas thegreen wavelength corresponds to approximately 550 nanometers inwavelength. Finally, the red wavelength corresponds to approximately 690nanometers in wavelength. It will be appreciated, of course, that theactual spectrum of lamp 105 may differ from spectrum 300. However, asknown in the art, the actual spectrum will still be concentrated in theR, G, and B wavelengths. It follows that white light beam 145 from lamp105 may be considered to primarily consist of the combination of R, G,and B beams. By rotating or rocking diffraction grating 140 about anaxis at, for example, point 155, the appropriate values of α and β areproduced such that diffracted beam 150 comprises either an R, G, or Bbeam. For example, by rocking diffraction grating 140 through an angularrange 160, diffracted beam 150 will sequentially comprise either an R,G, or B beam.

Advantageously, diffraction grating 140 introduces significantly lesspower loss into diffracted beam 150 than would be experienced if adichroic mirror or a color wheel were used to split white light beam145. In addition, no realignment means are required with respect toseparated images of R, G, and B beams, making manufacture of colorprojection system 100 simpler, thereby achieving cost savings. Also, theuse of a single LC microdisplay panel without the need for subpixelsbecause of the sequential projection provides additional cost savingsand manufacture simplification. Moreover, these significant advantagesmay also be achieved should diffraction grating 140 be a transmissiongrating such as grating 250 shown in FIG. 2 b. Because grating 250 is atransmission grating, diffracted first order beams 205 and 210 projectfrom the other side of grating 250 with respect to incident beam 200.Equation (1) still holds such that, in a color image projection system400 shown in FIG. 4 that incorporates a diffraction grating 140configured as a transmission grating, a diffracted beam 150 will stillsequentially comprise either an R, G, or B beam.

LC microdisplay panel 110 may comprise any suitable LC microdisplaypanel as is known in the art. However, in a projection televisionapplication, the sequencing of RGB images by LC microdisplay panel 110should be faster than the response time of human eyes. For example, toachieve cinema-quality projected images, a frame rate of 24 frames persecond is typically required. Each frame will consist of threesub-frames of RGB color (one red sub-frame, one green sub-frame, and oneblue sub-frame), which corresponds to an average of about 14milliseconds per color sub-frame at the 24 frames per second rate. Thus,LC microdisplay panel 110 should have a response time better than 14milliseconds to match modem film frame rates without the need forsoftware to perform interlacing or “pull-down” conversion. For example,consider the R, G, B color data used to drive LC microdisplay panel 110as shown conceptually in FIG. 5 with respect to two consecutive videoframes. The diffracted beam 150 is shown broken down into its sequentialR, G, and B components. To keep diffracted beam 150 synchronous withincoming RGB data, diffraction grating 140 (FIGS. 1 and 4) may be rockedsuch that diffracted beam 150 would provide the R, G, and B componentssynchronous with the incoming data as shown. It will be appreciated,however, that the sequencing of diffracted beam 150 may be adjusted tosuit a particular design and/or desired color adjustment. For example,consider again the spectrum 300 for an ultra high pressure lamp sourceshown in FIG. 3. Although the power is concentrated in the R, G, and Bwavelengths, the intensities are not equal. Specifically, the power forthe R wavelength is noticeably weaker than the power of the G and Bwavelengths. In addition, the sensitivity of human eyes to differentcolors is also wavelength dependent. Thus, diffraction grating 140 couldbe rocked such that diffracted beam 150 comprises a R beam for longerperiods than those periods in which diffracted beam 150 comprises a B orG beam. In other words, diffraction grating 140 would be moved such thatit stays within the appropriate angular range to form diffracted beam150 as an R beam longer than in the other angular ranges used togenerate the B and G beams. The time-sequenced color control of LCmicrodisplay panel 110 may be controlled by an ASIC or DSP processor(not illustrated) as is known in the art. Such a conventional ASIC orDSP processor may be further modified to also control the movement ofdiffraction grating 140. Alternatively, a separate processor, statemachine, or analog controller (not illustrated) could be used to controlthe movement of diffraction grating 140. It will be appreciated,however, that in a color slide or viewgraph projection application, theprocessing speed limitation of LC microdisplay panel 110 may be relaxedconsiderably.

Those of ordinary skill in the art will appreciate that color projectionsystems 100 and 400 are shown in simplified form in that numerousadditional components such as other polarization filters are necessaryto complete these systems. These additional components, however, areconventional and thus are not illustrated. To better provide a moreuniform beam power across the width of diffracted beam 150, a lensassembly 180 for focusing white light beam 145 onto diffraction grating140 may comprise a micro-lens array as described in co-pendingapplication entitled “Microlens Array”, U.S. Ser. No. 10/758,989, thecontents of which are hereby incorporated by reference. Such amicro-lens array may provide more uniformly distributed light intensityacross the whole field of the LC microdisplay panel 110 and thus improvethe light intensity distribution or brightness of projected images.

The movement of diffraction grating 140 may be accomplished usingconventional electric motors such as motor 700 illustrated in FIG. 6.Typically, diffraction grating 140 need only be approximately ½ squareinch in size and may be rocked through an angular range 160 of less than10 degrees to achieve the appropriate sequential diffraction of R, G,and B beams. Thus, the demands on motor 700 are fairly minimal, allowinginexpensive designs for motor 700 to be used.

Numerous modifications may be made to systems 100 and 400. For example,diffraction grating 140 may be replaced by a dispersive prism 705 asshown in FIG. 7 for a color image projection system 700. The remainingcomponents may be arranged as discussed for FIGS. 1 and 4. As is knownin the art, light refracted by prism 705 will refract differentlydepending upon wavelength. Thus, white light received from lamp 105 willbe separated into R, G, and B beams. By rocking prism 700 through anangular range 160, these refracted beams may be received sequentially byLC microdisplay panel 110 so that appropriate modulation may be applied.Advantageously, prism 700 may be constructed from rugged material suchas amorphous fused silica (synthetic quartz) that may be more resistantto oxidation than the materials used for diffraction grating 240. Itwill be appreciated that although prism 700 is configured as atransmissive prism, a reflective prism configuration may be used aswell.

In yet another embodiment, the single diffraction grating 140 shown inFIGS. 1 and 4 may be replaced by three separate gratings: a grating 805for red light, a grating 810 for green light, and a grating 815 for bluelight as shown in top and side views in FIGS. 8 a and 8 b, respectively.Each grating would be configured to diffract light at the same angle. AnLC microdisplay panel (not illustrated) may then be fixed to receive thediffracted light at this angle, whereby the LC microdisplay panel willsequentially receive R, G, and B diffracted light as gratings 805, 810,and 815 are laterally shifted with respect to a window 820 within a mask830. Gratings 805, 810 and 815 may be constructed in either a reflectiveor transmission configuration as discussed with respect to FIGS. 1 and4. A linear actuator (not illustrated) or other suitable motor may beused to laterally shift gratings 805, 810, and 815 at a desired ratewith respect to window 820. By changing the lateral movement speed asgratings 805, 810, and 815 move with respect to window 820, a particularcolor may be directed for a longer period at the LC microdisplay panel.Alternatively, the relative widths of gratings 805, 810, and 815 may bealtered to achieve the same effect. Moreover, a similar configurationmay be constructed be replacing the gratings with prisms.

Accordingly, although the invention has been described with respect toparticular embodiments, this description is only an example of theinvention's application and should not be taken as a limitation.Consequently, the scope of the invention is set forth in the followingclaims.

1. A color projection system, comprising: a lamp; at least onediffraction grating configured to diffract light from the lamp into adiffracted beam; and an LC microdisplay panel configured to receive thediffracted beam from the diffraction grating, wherein by moving the atleast one diffraction grating with respect to the light from the lamp,the diffracted beam received by the LC microdisplay sequentiallycomprises a diffracted red beam, a diffracted blue beam, and adiffracted green beam, the LC microdisplay panel being configured tosequentially modulate the diffracted red beam into a red sub-frame of animage, the diffracted green beam into a green sub-frame of the image,and the diffracted blue beam into a blue sub-frame of the image.
 2. Thecolor projection system of claim 1, further comprising: a motor to movethe at least one diffraction grating with respect to the light from thelamp.
 3. The color projection system of claim 2, wherein the motor isconfigured to move the diffraction grating by rocking the diffractiongrating through an angular range.
 4. The color projection system ofclaim 1, wherein the at least one diffraction grating comprises threediffraction gratings, each diffraction grating being configured todiffract a different color selected from the group of red, green, andblue.
 5. The color projection system of claim 1, wherein the at leastone diffraction grating is a reflection grating.
 6. The color projectionsystem of claim 1, wherein the at least one diffraction grating is atransmission grating.
 7. The color projection system of claim 1, whereinthe LC microdisplay panel is a reflective LC microdisplay panel.
 8. Thecolor projection system of claim 1, wherein the LC panel is atransmissive LC microdisplay panel.
 9. A method of color projectioncomprising: providing a light beam incident on at least one diffractiongrating; moving the at least one diffraction grating with respect to theincident light beam, wherein the movement of the diffraction grating issuch that a diffracted light beam from the at least one diffractiongrating sequentially comprises a diffracted red beam, a diffracted greenbeam, and a diffracted blue beam, wherein each diffracted beam isprojected onto an LC microdisplay panel, and sequentially modulating thediffracted red beam received by the LC microdisplay panel to form a redsub-frame of an image, modulating the diffracted green beam received bythe LC microdisplay panel to form a green sub-frame of the image, andmodulating the diffracted blue beam received by the LC microdisplaypanel to form a blue sub-frame of the image.
 10. The method of claim 9,wherein the movement of the at least one diffraction grating comprisesrocking the at least one diffraction grating through an angular range.11. The method of claim 9, wherein the providing act comprises providinga light beam incident on one of three diffraction gratings, eachdiffraction grating being configured to diffract a different colorselected from the group consisting of green, blue, and red, and whereinthe moving act comprises moving the three diffraction gratings withrespect to the incident light beam such that each diffraction gratingwill sequentially diffract the incident light beam into its selectedcolor.